Are double-vented jackets required on the "A" circuit?
No, they are not, but jackets designed for the hunter and jumper rings do tend to be double-vented. Single-vented jackets tend to be designed for the dressage ring. Dressage jackets usually have metallic buttons that might not stand out in a good way in the conservative hunter ring. Dressage jackets are also cut differently and might be a bit more difficult to jump in. There is certainly no rule against using a single-vented jacket.
Does your horse need boots in the jumpers?
Not necessarily. Some horses jump better without boots and are shown with naked legs, even at the top levels. Boots tend to be used for protection, in case a horse accidentally strikes the back of a front leg or interferes. They are often used as a precaution, not because the horse needs boots, but just in case something happens. There is no rule that horses must compete in boots in the jumper ring.
What are the rules for shirt colour in equitation classes?
The classic shirt colour for equitation classes is white. There is no rule, however, that requires this in the Equine Canada rule book (or in the USEF rule book, as far as I am aware).
How do you reverse in a hunter under saddle class?
You should make a small half-circle when asked to reverse. This can be done in one of two ways. The first possibility is to turn to the inside and keep turning in that direction until you reach the track on the new rein, and change the bend. The other is to turn to the inside, and then change the bend and turn back the other way in order to spend more time on the new bend.
The more correct method is the second, since it gives you a better chance to establish the new bend and flexion. The proper term for this second diagram is a half turn in reverse, while the first would just be a half turn.
Why not bang hunter tails?
There is no rule against banging the tail of a hunter ("banging" means cutting the tail bluntly), and I have seen several hunters win with their tails banged. Those hunters who do show with a banged tail usually still have quite a long tail compared to your average jumper. The natural tail would probably be considered a more traditional look, and might add to the smooth image of a hunter, while the banged tail might draw the eye away from the horse. A natural tail can sometimes look quite thin, however, and banging can give the appearance of more thickness. While the natural look is more common, you certainly wouldn't get thrown out of the ring for banging a tail.